Unable to download Big Sur update

pilchardcat wrote:

My Wifi isn't the fastest so whether it could be that? I haven't ever had any problems though!

yes, wi-fi can indeed be an issue. even for people that have fast speeds. it's often recommended here to use a wired ethernet connection to download macOS updates/upgrades. the files are rather large and can become corrupted or not complete at all over wi-fi. this is especially true for the installer of big sur. it's transferring WAY MORE data as compared to previous upgrades.

Try Shut down the Router. Shut down the MBA. Restart the Router. Move MBA as close to router as possible and Startup in Safe Mode - Shift - key right away. Loads slowly - Normal. Disconnect AL other devices using the Wifi - want the Maximum bandwidth for the MBA. Open the Apple Apps Store and commence the download of Big Sur. Once downloaded if it tries to Launch - Command + q - quit. Now, make a Time Machine Backup and if possible copy the New Application in Apps Folder - Install Big Sur to safe External Drive - good to make bootable Installer later.

Now, launch the installer

See if it will download while you are in Safe Mode.

How to use safe mode on your Mac - Apple Support